app: reorganize a bit gimp_widget_load_icon().
Massimo spotted some warning with clang in #1608 about pixbuf being used
initialized. Rather than just initializing it, I am actually
reorganizing a bit more the function because there was a bit of a logics
bug. In some weird case, it would have still been possible for this
function to return NULL instead of a magenta square (the case was: the
icon was not present in the icon theme; then wilber-eek was either not
present or failed to load).
This new code organization is more clearer, as a step by step, should
better identify the various failure cases and always return an allocated
GdkPixbuf.